1. Field of the Invention
This invention relates to a fuel supply control system for internal combustion engines, and more particularly to a fuel supply control system for internal combustion engines, which controls the air-fuel ratio of an air-fuel mixture to be supplied to the engine so as to improve the purification rate of a catalytic converter arranged in the exhaust system of the engine.
2. Prior Art
Conventionally, there is widely employed an air-fuel ratio control method for internal combustion engines, which detects the concentration of oxygen present in exhaust gases from the engine by an oxygen concentration sensor (hereinafter referred to as "the O2 sensor") or a linear output air-fuel ratio sensor (hereinafter referred to as "the LAF sensor") arranged in the exhaust system of the engine, and feedback-controls the air-fuel ratio of an air-fuel mixture to be supplied to the engine to a desired value in response to the detected oxygen concentration.
In recent years, an air-fuel ratio control system has been used, which adjusts a desired air-fuel ratio of a mixture to be supplied to the engine, an inversion delay time of an O2 sensor arranged in the exhaust system at a location downstream of a catalytic converter, etc. based on an output from the O2 sensor, to thereby finely adjust a control amount for the fuel supply amount, determined in response to an output from an O2 sensor arranged upstream of the catalytic converter, whereby the air-fuel ratio of the mixture can be stably controlled and also exhaust emission characteristics of the engine can be improved.
On the other hand, a method has been proposed, for example, by Japanese Provisional Patent Publication (Kokai) No. 2-11841, which forcibly oscillates the air-fuel ratio of a mixture to be supplied to the engine in order to enhance the purification rate of a catalytic converter arranged in the exhaust system. Further, a method has been proposed, for example, by Japanese Provisional Patent Publication (Kokai) No. 4-131762, which compares an actual air-fuel ratio detected by an O2 sensor arranged downstream of a catalytic converter in the exhaust system with an air-fuel ratio value calculated from an O2 storage amount obtained from a simulation modeled on the catalytic converter, and judges that the catalytic converter is deteriorated when the two air-fuel ratio values are largely different from each other.
However, the above proposed methods still require further improvements for the following reasons: That is, the proposed air-fuel ratio control methods merely control the air-fuel ratio to a predetermined desired value, but do not contemplate positively enhancing the purification rate of the catalytic converter to the maximum by controlling the air-fuel ratio. In addition, the purification rate of the catalytic converter largely depends on parameters representative of the temperature and deterioration degree of the catalytic converter. However, the proposed methods do not detect or estimate these parameters to use them for controlling the air-fuel ratio. As a result, the purification rate of the catalytic converter cannot be maximized in various operating conditions of the engine. Besides, recent studies have disclosed that the purification rate of the catalytic converter varies depending on the amplitude and repetition period of the air-fuel ratio of exhaust gases even if the air-fuel ratio assumes the same average value. This is because it is considered that the purification rate of the catalytic converter largely depends on an O2 storage amount therein. However, no method has been proposed, which controls the air-fuel ratio of the mixture by detecting or estimating of the O2 storage amount of the catalytic converter and reflecting the detected or estimated O2 storage amount upon the manner of air-fuel ratio control.